1. Technical Field of the Invention
The present invention relates to a cellular telephone network implementing a smart antenna technology in addition to sector antenna technology at its base stations and, in particular, to a hand-off procedure for use in connection with such a network, wherein the handoff procedure preserves the cell borders defined for each cell by sector antenna operation.
2. Description of Related Art
It is well known in the art to utilize directive antennas in cellular communications networks. The most commonly recognized example of directive antenna use in cellular communications networks is based on the principle of sectorization, as is illustrated in FIG. 1. A cell site 10 may comprise either one omnidirectional cell or a plurality, for example, three (or more), sector cells 12. Directive antennas 14, each with an appropriately selected beamwidth for the sector cell 12, are then utilized at each base station 16 to form a plurality of wide beams 18, one per sector cell, with the totality of the beams formed thereby providing substantially omni-directional radio frequency coverage throughout the cell site area. In operation, each of the formed wide beams 18 is in continuous use to provide service within each corresponding sector cell 12.
Another example of directive antenna use in cellular communications networks is based on the use of smart antenna technology, as is illustrated in FIG. 2A. Directive antennas 20 are utilized at each base station 16 of a cell site 10 to form a plurality of separate, perhaps slightly overlapping, narrow beams 22 within each sector cell 12, with the totality of the beams formed thereby providing substantially omni-directional radio frequency coverage throughout the cell site area. In operation, and in contrast to the operation of the sectorized beams 18 of FIG. 1, the narrow beams 22 are intermittently used only when necessary to provide service to one or more mobile stations 24, as is illustrated in FIG. 2B. Put another way, in smart antenna technology, the base station 16 controls its directive antenna 20 to activate at any given time only those individual ones of the plurality of separate, perhaps slightly overlapping, narrow beams 22 as are needed to serve active mobile stations 24 within the cell site 10.
Reference is now made to FIG. 3 wherein there is shown a diagram of directive antenna beam coverage from adjacent cells 12. Suppose a first cell 12(1) includes a directive (sector) antenna 14, having an appropriately selected beamwidth for the sector, at its base station 16(1) that is operable to provide a sector coverage beam 18 to serve a mobile station 24. Suppose further that a second, adjacent, cell 12(2) includes a directive (smart) antenna 20 at its base station 16(2) that is operable to form, at any one time, a narrow beam 22 to serve a mobile station 24. The theoretical (or ideal) hand-off border 26 between the first cell 12(1) and the second cell 12(2) would lie approximately half-way between the base stations 16 for the respective cells 12 and would be defined by approximate interaction of the sector coverage beam 18 in the first cell and a theoretical (or perhaps physical, if present) sector coverage beam 18' in the second cell.
It is noted here that the theoretical border 26 is illustrated in the manner of a zone between lines 26(1) and 26(2) to account for the fact that hysteresis values, as is well known in the art, affect the relative location between the base stations where hand-off would actually occur. More specifically, because of the introduced hysteresis value, which must be met by the signal strength measurements made with respect to, and compared between, the two cells 12, a mobile station 24 moving from the first cell 12(1) into the second cell 12(2) would not actually achieve a hand-off until at least reaching line 26 (2). Conversely, a mobile station 24 moving from the second cell 12(2) into the first cell 12(1) would not actually achieve a hand-off until at least reaching line 26(1).
One advantage of the use of smart antennas 20 is the extended range of coverage, as generally indicated at 28, obtained when compared to the range of coverage provided by sector antennas 14. One consequence of this extended coverage range 28 is a disturbance in location of the cell border 26, as generally shown at 30. A number of drawbacks arise from such a disturbance 30 in the cell border 26. First, the base station 16(2) tends to provide service to mobile stations 24 which are not located within or near its cell 12(2), and thus service load between the cells is not properly shared and system management issues become too complex. Second, the base station 16(2) downlink broadcasts made from the second cell 12(2) in order to provide service to a distant mobile station may be made at such a high power level as to inject downlink co-channel interference into other cells within the network which reuse its same frequency. Third, distant mobile station 24 uplink broadcasts may be made at such a high power level as to more quickly and unnecessarily drain battery life. Further, because the mobile station is in the vicinity of a cell other than cell 12(2), uplink co-channel interference will also increase in the reuse cells.
There is accordingly a need for a hand-off technique that accounts for the extended coverage range 28 of smart antennas 20 in comparison to sector antennas 14 and thus corrects for any introduced disturbance 30 in location of the cell border 26 in making the hand-off determination.